<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>类选择器使用案例</title>
    <style>
        .red { background-color: red; width: 200px; height: 200px;}
        .green { background-color: green; width: 200px; height: 200px;}
    </style>
</head>
<body>
    <div class="red"></div>
    <div class="green"></div>
    <div class="red"></div>
</body>
</html>